1. Field of the Invention
This invention relates to herbicidal compositions in liquid form having extended penetrating and systemic activity upon application to susceptible flora. The phytotoxic agent is resistant to initial plant outer cell layer burn that would interfere with its activity but also has been found to be free of such cell damage properties during the extended period of exposure of the plant to the active agent which is a major feature of its total efficacy since the product also resists crystallization for an extended period under normal ambient conditions after application. These desirable characteristics are realized in part because the pH of the material is at least 6 or above at the time of application and remains at such level long periods of time involving many days thus preventing reversion of the toxicant to an acid form which could adversely affect the surface cellular structure of the plants and thus impede the permeation of the material into the plants as well as translocation thereof to the root system.
2. Description of the Prior Art
Acidic herbicides such as 2,4-dichlorophenoxyacetic acid have long been used to control unwanted vegetation. 2,4-D as it is generally known is an insoluble crystalline material having a pH of approximately 2. For convenient application, it is normally converted to liquid form by conversion to water soluble salts or emulsified esters. The ester formulations have been found to be more effective than the salts in the control of noxious vegetation but have the unwanted characteristic of migrating to adjacent desirable vegetation because of the volatility thereof with unacceptable resultant damage.
Efforts to solve the volatility problem including preparation of water soluble salts such as the dimethylamine salt of 2,4-D have not been totally satisfactory because upon volatilization of the amine the herbicide reverts back to its initial acidic form. The acidic composition on the plant surface damages the plant cells thus forming a mechanical barrier of dead tissue which prevents effective penetration of the herbicide into the plant and translocation to all of the above ground and below ground segments.
Canadian Patent No. 879,602 of Ciba-Geigy Canada Ltd. dated Aug. 31, 1971 describes relatively non-volatile herbicidal compositions produced by reacting amines, ammonium, sodium, potassium and lithium compounds with acidic compounds containing for example acid moieties preferably comprising halogenated carboxylic acids, aromatic carboxylic acids, and the halogenated aromatic carboxylic acids, with particularly preferred groups being derivatives of phenoxy acids, and derivatives of benzoic acids. Other groups of compounds are defined as including organo arsenic acids, as well as halogenated aliphatic carboxylic acids. In its preferred embodiments, the compositions of the Canadian Patent involve the mixture of two or more different salts containing the different salts in varying proportions. One or more surfactants are described as being added to the composition causing the liquid to be relatively non-volatile at normal ambient temperature conditions.
Although the formulations of the referenced Canadian '602 Patent did exhibit the desired low volatility, the pH of the herbicidal compositions was sufficiently on the acid side to cause the material to in effect burn the surface layers of the plants on which the herbicide was applied thereby creating a barrier to phytotoxic activity of the herbicide and limiting its effectiveness over desired extended periods of time. As a consequence, the compositions disclosed in the '602 Canadian Patent, although in commercial use, do not lend themselves to use in many aerial applications where extended activity under varying ambient conditions is an essential property of the composition and suffer from the defect of causing plant burn in many instances which significantly reduces its activity in a relatively short period of time after application.
A herbicidal composition especially designed for agricultural spraying is described in Guth U.S. Pat. No. 3,764,293 of Oct. 9, 1973 wherein the patentee suggests that acidic herbicidal agents reacted with metal hydroxides to produce salts are combined with polyhydric diols to assist in dispersion of the herbicidal material in the base water. Here again, the formulation although clearly capable of exhibiting initial herbicidal activity had the tendency to burn the surface cellular structure of susceptible plants which limited activity of the material over extended periods of time and required more frequent application of the phytotoxic agent than is desirable.